This invention relates generally to pulse conditioning circuits and, more particularly, to pulse conditioning circuits used to convert transducer analog pulses for driving electronic digital processing circuitry.
In various electronic digital control or monitoring applications for a rotating device or system variable, a sensing element is used which produces a repeating analog signal whose amplitude and/or repetition rate varies with changes in the speed of the device or the system variable. For example, a variable reluctance magnetic pickup coil, known as a Fidler coil, is used to sense rotor speed in a gas centrifuge for separation of isotopic gaseous mixtures. The coil is located in the field of a magnetic bearing for the rotor. A differential signal is induced in the coil each time a field perturbating notch formed in the bearing magnetic armature carried by the rotor passes the coil. The signal obtained is not ideal for driving electronic digital processing circuitry since the amplitude and pulse width undergo changes of orders of magnitude over the normal speed range of an operating centrifuge. The signal slope is very low and noise levels at run speed are greater than signal levels at low speed.
Since this type of analog signal cannot be used to drive a digital control or monitoring system directly, it must be conditioned to provide a digital timing signal. This timing signal is usually provided by a one-shot in which the leading edge of the digital pulse output coincides with the negative-going zero crossing of the analog input pulse.
Several circuits have been developed to condition the Fidler signal to obtain the digital timing signal. Generally, these circuits employ an adaptive sensing technique in which a DC level is developed proportional to the input peak voltage and used to dynamically vary the threshold at which the one-shot is armed by the input signal. This allows the circuit to reject noise components at the input in situations where the high speed noise is greater than the low speed signal amplitude.
These circuits employ a single pole low-pass input filter to eliminate high-frequency noise components outside the effective bandwidth of the signal spectrum. These circuits use an input differential amplifier to change the differential input signal to a ground referenced signal and reject common mode noise. The shielded twisted-pair cable from the coil is terminated in a 100 ohm input resistance to reduce differential induced noise.
In amplifying and conditioning a sensor signal, such as the signal from the Fidler coil, noise may be introduced into the signal producing a pulse-to-pulse variation in the relationship between the digital timing signal pulse and the zero crossing of the magnetic pick-up signal. This pulse-to-pulse variation, called jitter, places fundamental limits on the accuracy and response time of the speed and acceleration processing circuitry. In order to maximize the accuracy and/or minimize the response time of the processing circuitry, the jitter must be minimized. Thus, there is a need for improvements in pulse conditioning circuits to provide more accurate timing pulse generation for use in digital processing circuitry.